For example, following development of a higher degree of integration of a semiconductor integrated circuit or the like, a photomask used in a micromachining process contained in a semiconductor manufacturing process is required to have a high pattern accuracy.
In photomasks currently used, a chromium-based material is generally used as a light-shielding film in view of machinability of a high-accuracy pattern.
However, such a demand for a higher-resolution pattern of the photomask following the development of a higher degree of integration of the semiconductor integrated circuit can not be met by an existing patterning method of patterning a chromium-based light-shielding film using a resist pattern as an etching mask. It has been revealed that, in this method, as a fine opening pattern (hole) has a higher resolution, an influence of degradation in size and shape of the fine opening pattern (hole) due to a microloading effect can not be neglected and becomes an obstacle in practical use.
Specifically, as the existing patterning method of patterning the chromium-based light-shielding film by the use of the resist pattern as the etching mask, use is mainly made of a method of forming a Cr pattern by dry etching using a gas composition predominantly containing a Cl2+O2 mixed gas and using a resist pattern on a Cr film as a mask (see Japanese Unexamined Patent Application Publication (JP-A) No. 2001-183809).
For the dry etching, an RIE (Reactive Ion Etching) apparatus is typically used. However, in order to meet the recent demand for finer patterns and a higher pattern accuracy, an ICP (Inductive Coupling Plasma) system is considered (see “SPIE”, Vol. 3236, C. Constantine et al, 1997, pp. 94-103, hereinafter referred to as the first prior art).
In this case, etching is generally performed at a high plasma density using an ICP power higher than an ICP power which causes electron density jump (i.e., a condition achieving stable plasma discharge) (see Journal of “SPUTTERING & PLASMA PROCESSES”, Vol. 13, No. 4 (entitled: “Production and Physics of High-Density Plasma”, written by: Hidero Sugai, p. 7), Oct. 9, 1998, published by Sputtering and Plasma Technology Department of Japan Technology Transfer Association, hereinafter referred to as the second prior art).
However, the first and the second prior arts mentioned above have three problems which will be described hereinunder.
As a first problem, there is a large difference in size between a resist pattern after development and a Cr pattern after etching (hereinafter referred to as a conversion difference between a resist and Cr or simply as a conversion difference). Heretofore, use has been made of a method in which development and etching conditions anticipating the conversion difference are adopted to improve an accuracy with respect to designed pattern data.
However, in recent years, a pattern having a fine and complicated shape, such as an optical proximity correction (OPC) pattern, is used. Further, there is a demand for high-accuracy formation of a pattern having a size difference and a density difference in a mask plane. Under the circumstances, it is difficult to form a high-accuracy pattern by using the existing method.
Specifically, Cr recession by isotropic etching of the resist causes the following problem. Comparing square patterns of the same size, the square opening pattern (removed portion) is increased in size and its corners are rounded. On the other hand, the square shielding pattern (remaining portion or Cr portion) is decreased in size and its corners are kept at substantially right angles. This results in differences in size and in corner shape between these patterns.
The above-mentioned problem exerts the following influence on a mask manufacturing process and a mask quality. First, the corners of the square opening pattern being rounded induces occurrence of a suspected defect, resulting in a serious obstacle to an inspection process. Further, the pattern shape is not formed in exact conformity with designed pattern data, resulting in reduction in margin in a lithography process in a semiconductor manufacturing process and a large number of steps required in condition setting. Further, the above-mentioned conversion difference becomes an obstacle to fine pattern formation on the mask. It is possible to accommodate the above-mentioned conversion difference by data sizing. In this case, however, the sizing amount increases so that a conversion time increases.
As a second problem, because of the microloading effect, the conversion difference between the resist and Cr widely changes depending on the size of the opening pattern (hole). As the opening pattern is a finer opening pattern having a smaller size, a finished size is smaller and the conversion difference has a greater absolute value. The relationship of a shift amount from a designed pattern size with respect to a change in designed pattern size is called CD linearity. If a change in shift amount from the designed pattern size is large with respect to the change in designed pattern size, the expression that the CD linearity is inferior is used.
The change in shift amount from the designed pattern size being large with respect to the change in designed pattern size represents that variation in conversion difference between the resist and Cr is large. This problem exerts the following influence on the mask manufacturing process and the quality.
At first, because of this problem, the conversion difference varies depending on the size of the opening pattern so that CD accuracy (particularly, the CD linearity) is degraded. This results in reduction in margin in the lithography process in the semiconductor manufacturing process and a large number of steps required in condition setting. The reason is as follows. Although the CD accuracy may be compensated by another process such as exposure, a condition therefor may not be an optimal condition considering other CD accuracy than the CD linearity.
As a third problem, the sectional shape of Cr depends on the size of the opening pattern (hole). If the opening pattern is decreased in size to become a fine opening pattern, the sectional shape is tapered. This problem exerts the following influence on the mask manufacturing process and the quality.
At first, because of this problem, the sectional shape of Cr may be changed in the plane. In this event, due to an electromagnetic optical effect, an optical size variation as large as several times a variation resulting from the sectional shape is caused to occur. This results in reduction in margin in the lithography process in the semiconductor manufacturing process and a large number of steps required in condition setting. Further, in case where the length of the mask is measured by an optical length meter, length measurement accuracy is degraded.
It is therefore a first object of the present invention to provide a method of manufacturing a photomask, which is capable of reducing a conversion difference regardless of the shape of a pattern (opening pattern (hole), light shielding pattern (dot), line and space, etc.), a size difference, or a density difference (particularly regardless of hole or dot) and of reducing a difference in shape between a hole or a dot.
It is a second object of the present invention to provide a method of manufacturing a photomask having excellent CD linearity (constant conversion difference) even if a hole has a small size.
It is a third object of the present invention to provide a method of manufacturing a photomask in which the sectional shape of a hole does not depend upon the size and is excellent even if the size is small.